The preparation of organic sulfonic acids and of organic sulfonates for use as soaps, detergents and additives for lubricating oils is well known in the art. In the past, the sulfonation of alkylaromatic hydrocarbons has generally been carried out in batch operations. In these processes it was customary to employ a number of steps, the first of which was to sulfonate the hydrocarbon by treating it with a large excess of oleum in order to drive the reaction to completion. The second step was to dilute the mixture with water and stratify it in order to separate the sulfonic acid from the excess diluted sulfuric acid. The final phase consisted of converting the sulfonic acid to the sulfonate by neutralization. Due to the time factor involved, efforts have been made to develop continuous sulfonation processes, such as one in which the acid flows counter-currently to an excess of the hydrocarbon in liquid phase, and another in which an excess of preheated acid flows counter-currently to a stream of the hydrocarbon vapors. Up until the present time, however, a continuous process for the sulfonation of such hydrocarbons has not been considered commercially practical. One of the major drawbacks has been the buildup of sludge (residual sulfuric acid) which is a disfavorable substance in these particular reactions. Attempts have been made to cut back on the formation of this residual sulfuric acid by employing sulfonating agents other than oleum but but with little success. Although the presence of sulfuric acid in the product could be eliminated, the resulting product would still exhibit a number of undesirable characteristics. An example is the use of liquid sulfur trioxide as the sulfonating agent in which the resulting product exhibited a pH drift to the acid side after apparent neutralization and also contained an off odor. In our quest to formulate a quicker and more favorable process we have invented a continuous process for the sulfonation of aromatic hydrocarbons which also has the desirable characteristic of little or no sludge formation.
The term "alkyl benzene sulfonic acid" as used throughout this application can encompass almost any alkylaromatic and includes such sulfonic acids as polypropylene benzene sulfonic acid and polybutene-1 benzene sulfonic acid. The polypropyl radical of the polypropyl benzene sulfonic acid generally has a molecular range selected from approximately between 350 and 1,200.
The polybutene-1 radical of the polybutene-1 benzene sulfonic acid preferably has a molecular weight range of about 350 to 600. Currently, many of the processes taught by the prior art describe a method whereby the liquid sulfur trioxide sulfonating agent is diluted with an inert gas in the initial step. Contrary to these procedures, our system does not necessitate this dilution action. It should also be noted that our sulfonator - static inline mixer -- approximates plug flow in a manner facilitating the reduction of possible sludge formation. It is significant in that since there are no moving parts in the sulfonator -- static inline mixer, the capital and operating costs are relatively low.
"Static inline mixer" as used throughout this application refers to a motionless mixing device or static mixer such as the Kenics motionless inline mixer or the Koch mixer. Our experimental tests were conducted using Kenics static mixers and these particular mixers are described in the following publications:
(1) U.S. Pat. No. 3,286,992, issued Nov. 22, 1966. PA1 (2) The article entitled "Motionless Inline Mixers Stir Up Broad Interest" appearing in the May 19, 1969 issue of Chemical Engineering.
A view of the prior art includes use of gaseous sulfur trioxide diluted with inert gas in falling film reactors (U.S. Pat. Nos. 3,328,460 and 3,529,645); batch process sulfonation (U.S. Pat. Nos. 3,056,831 and 3,248,413); countercurrent extraction processes with or without sulfuric acid present (U.S. Pat. Nos. 3,677,934 and 3,798,261); sulfonation in the presence of inorganic sulfites (U.S. Pat. No. 3,789,067) and processes requiring the presence of 10% sulfuric acid (British Pat. No. 669,899, issued in 1952). None of the processes of the prior art involve the use of a sulfonator, which is a "static inline mixer" in the manner employed in our process.